We examined genetic molecular approaches to investigate mechanisms determining the adaxial and the abaxial sides of leaves, and proposed a hypothesis which succinate semialdehyde has a role in the mechanism. Second, found that the boundary separating cells expressing either the adaxial or the abaxial side-specific genes moved as the leaf grew. We proposed a mathematical model explaining the boundary shift. Third, we examined key genes necessary for lateral growth of leaves, and found that the genes are expressed newly-identified middle domain of the developing leaves.